A paper or web coming out of a paper machine has rough surfaces, which require finishing for most purposes of use. For finishing, smoothing devices (e.g. machine calenders) and resilient-nip calenders (e.g. soft or supercalenders) are known in the art and the smoothing devices comprise hard rolls only, and smooth the paper surfaces substantially in one plane. The nips in a resilient-nip calender are called soft nips since a hard roll forms a pair with an elastically resilient roll. In a resilient-nip calender, the elastically resilient rolls are, as a rule, paper rolls, whose surface layer consists of paper rings fitted as layers one above the other. A resilient-nip calender also contributes to smoothing to a certain extent, but, primarily serves to impart a glaze, i.e. the surface of the paper web is compacted and closed.
Instead of softcalendering, the term "matt calendering" can be used. A supercalender is an off-machine device, whereas a soft calender is an on-machine or on-line device (as a rule having 1 or 2 nips) or an off-machine device (having up to 4 nips). In soft calenders the resilient rolls are not paper rolls, as they are in supercalenders, but they are different types of polymer or equivalent rolls, whose own internal generation of heat is lower than in paper rolls and whose sensitivity to surface damage is lower. As a rule, softcalendering is calendering that is carried out as an on-line operation while making use of high temperatures (clearly higher than the temperatures in a supercalender) with a minimum number of nips. Currently, a soft calender is used extensively instead of a machine calender with matt qualities as well as with coated papers in connection with a paper or coating machine when either the running speeds are low, the machines are narrow, and/or the linear loads and/or temperatures employed are not among the highest. As a rule, high-gloss papers continue to be calendered by means of a supercalender.
On-line operation of a calender imposes particular requirements on the calender as compared with separate supercalenders. The most important requirement is that a minimal proportion of the paper becomes "broke" because of particular operations and disturbance in the calender. One such operation is the threading of the web. Most disturbances in operation result from damage to rolls, in particular to soft rolls, and from standstills caused by such damage.
At present, soft calendering is mainly used as a substitute for machine calendering only. For calendering of qualities of higher gloss, the present roll materials are not sufficiently durable for on-line calendering, i.e. with two nips at the speed for a paper or coating machine. Since the roll materials do not withstand the conditions under which a quality product can be achieved, one means of achieving a quality product is to increase the number of nips. If attempts are made in the future, to substitute for the supercalender, tests that have been carried out indicate that by means of fewer than four nips, with the present-day running speeds, do not result in the supercalendering quality needed for present bulk qualities, such as SC- and LWC-papers. Efficient SC- and LWC-machines are wide high-speed machines (&lt;1400 m/min), requiring a simple threading of the web free of disturbances. In the prior art, soft calenders that are on-line connected to a paper machine consist of units subsequent to the machine that comprise a soft calender roll and a hard calender roll, a necessary number of such units being placed one after the other so that the web to be processed runs substantially horizontal. In these calenders the threading of the web is relatively easy and free of disturbance, because the subsequent nips can be opened and the end of the web can be passed through the calender as a substantially horizontal straight run. It is a drawback of said calender that the calender takes quite a large space in the direction of running of the web, which increases the cost of the machine hall.
It is a further drawback that the embodiment of the calender with separate frames becomes quite expensive.
In the prior art, on-line calenders are also known in which there are calender modules consisting of three calender rolls placed one above the other. In these modules, the middle roll is a hard roll and soft rolls are placed at both sides of the hard roll, so that in each module two calender nips are formed, placed one above the other. Said hard roll is journalled as fixed in the calender roll, and the soft rolls are displaceably arranged and supported by loading arms for the opening, relieving and loading of the nips. Since in an on-line calender more than two nips are needed, in said construction a three-roll module of the sort described is provided at each side of the vertical frame of the calender. The construction of said on-line calender is relatively compact and takes little space, but it involves the problem of difficulties in threading through a great number of curves as well as of the standstills resulting from breakdowns of the soft rolls which are sensitive to damage.
As was stated above on-line soft calenders have, as a rule, comprised 1 or 2 nips only, because in on-machine operation, with a higher number of nips and with prior-art solutions, the threading of the web has caused difficulties for which the prior-art calenders have not provided solutions. Even though, above and below, for the sake of conciseness, on-line calenders of paper machines have been described, it should be emphasized that the scope of the invention also includes on-line calenders of paper coating machines and of corresponding paper finishing machines separate from paper machines, in which calenders with substantially the same problems and needs for development occur as in said calenders of paper machines.
An object of the present invention is to avoid the drawbacks described above and to provide such an on-line calender which takes relatively little space in the machine direction, so that the calender can be accommodated in the usually limited space that is available, for example, when a machine calender is being modernized.
Another important object of the invention is to provide an on-line calender wherein the threading is free of disturbance and free of problems, the objective being that the threading should take a minimum of time during which the paper broke coming out of the paper machine or finishing machine is passed.
It is a further object of the invention to provide a calender construction whose cost of manufacture is favorable and in which, it is possible to use calender modules of more or less standard construction, by which modules it is thus possible to assemble different on-line calenders that have a sufficiently high number of calendering nips in compliance with the requirements imposed by the product.